The present invention relates generally to circuits for supplying drive current to lasers or other optical sources, and more particularly to load detection circuits for use in an output stage of a laser driver or other optical source driver.
Laser diodes and other types of semiconductor lasers are in widespread use as optical sources in high-speed optical data transmission applications. Laser diodes are particularly desirable in such applications due to their high optical output power and spectral purity. A laser driver circuit, also referred to herein as simply a xe2x80x9cdriver,xe2x80x9d is used to supply appropriate drive current to a semiconductor laser, so as to control the optical output signal between an xe2x80x9conxe2x80x9d state corresponding to a logic one level and an xe2x80x9coffxe2x80x9d state corresponding to a logic zero level, in accordance with the data to be transmitted.
Conventional semiconductor laser driver circuits are described in U.S. Pat. No.5,883,910, issued Mar. 16, 1999 in the name of inventor G. N. Link and entitled xe2x80x9cHigh Speed Semiconductor Laser Driver Circuits,xe2x80x9d which is incorporated by reference herein.
A significant problem with conventional semiconductor laser driver circuits such as those described in the above-cited U.S. Pat. No. 5,883,910 is that such circuits can be damaged as a result of output stage saturation. For example, in the process of manufacturing optical systems that use laser drivers, components may be inadvertently left off a laser driver circuit board during assembly. Such a condition is usually discovered only when testing the laser driver board after assembly. However, at this stage in the process, a missing component can result in an undesirable saturation of the laser driver output stage. The saturation of the laser driver output stage can destroy the laser driver on the spot, or may simply damage it such that the driver will be susceptible to failure in the field. Another scenario that can lead to the saturation of the laser driver output stage is the use of a faulty load board connection. Such load boards are commonly used in factory testing of a laser driver, prior to shipment of the driver to a customer, and saturation resulting from an improper connection on a load board can cause the driver output stage to be damaged or destroyed.
Previous attempts to address the output stage saturation problem described above include limiting the output stage fault current by adding a current limiting resistor in the base circuit of one or more output stage transistors. Unfortunately, adding a resistance large enough to provide the desired protection can significantly limit the maximum operating speed of the driver, and may also cause other portions of the output stage circuitry to enter saturation in the presence of the fault condition. Another possible approach is to increase the size of the collector and emitter of one or more of the output stage transistors, so as to accommodate the substantially higher current loads associated with the fault condition. However, the additional parasitic capacitances associated with this xe2x80x9cbrute forcexe2x80x9d approach will prevent proper operation at high speeds, e.g., operating speeds on the order of 2.5 Gbits per second or more.
A need therefore exists for improved driver circuits, for use with semiconductor lasers and a, other optical sources, which are configured to prevent the above-described output stage saturation problem without limiting the operating speed of the driver circuit.
The invention provides improved optical source driver circuits which meet the above-noted need.
In accordance with one aspect of the invention, a driver circuit for a laser diode or other optical source includes an input stage, an output stage, a current generator circuit and an output load detection circuit. The current generator circuit is adapted to establish a modulation current for application to one of a first output and a second output of the output stage in accordance with a differential or single-ended input data signal applied to the input stage. The output load detection circuit has first and second inputs coupled to the respective first and second outputs of the output stage, and is configured to detect an improper load condition at one or more of the first and second outputs of the output stage and to generate a corresponding output indicator. The output indicator is utilized in the driver circuit to control the modulation current so as to prevent saturation of the output stage in the presence of the improper load condition. For example, the output load detection circuit may be configured to determine if a voltage level of at least one of the first and second outputs of the output stage drops below a designated load detection sense threshold, in which case the modulation current can be disabled or otherwise interrupted so as to prevent the saturation of the output stage.
Advantageously, the invention can prevent undesirable saturation of the output stage of a laser driver under fault conditions, while also maintaining the ability of the laser driver to operate at high speeds.